PC Dual casting on PC alteration oak spell

How I am suppose to make it work I don't see the difference does dual casting require more magic. If I press the right and left key is show like I need more magic how much will oak spell needs for dual casting?

Dual casting a spell costs 2.8 times the amount of magicka for a 2.2 times magnitude increase. In Alteration, the time of the spell is affected, not the level of flesh protection. So a single cast Oakflesh lasts for 60 seconds, and a dual cast one will last for 132 seconds. If your Oakflesh costs 40 Magicka to cast, a dual cast one will cost 40*2.8 = 112 Magicka to cast. If you don't have that much, you won't be able to dual cast it. If you successfully dual cast the spell, you can go into your active effects menu and see how much time is remaining.

When dual casting a spell, you hit both of the triggers at the same time, and instead of two glowing hands charging the spell, you should see one larger glowing ball in between the two hands.